All current aircraft airframe manufacturing techniques require structural shimming at interfacing surfaces where tolerance build up prevents net line fits. The current technique is to hand cut laminated shim material to create a shim sized to fill the gap between the interfacing surfaces. After hand fitting and contouring is completed the shim profile is trimmed with an aircraft shears. Because this technique is time consuming and labor intensive, it is undesirably expensive. It is also undesirable because the technique depends on the skill of the person that manufactures the shim, which varies from individual to individual.
Various proposals have been made to overcome the disadvantages associated with the foregoing technique. One proposal has been to form shim master molds and use the result to create the necessary shims. While useful in some situations, this technique is not useful in other circumstances, particularly where the gap to be filled by the shim is very narrow. Further, the master mold shims have to be hand tailored for each individual interface situation.
In order to overcome the foregoing and other disadvantages, proposals have been made to measure the gap between the surfaces to be shimmed and use the resultant information to create a shim. One prior art device for measuring interface gaps is a relatively large electromechanical tool. The tool includes a two-piece probe having lips that are suitable for insertion into the gap whose thickness is to be measured. The measuring portion of the device includes an electric motor coupled to the probe for moving the probe pieces apart, and a shaft angle encoder for measuring the separation between the tips of the probe. In addition to being an undesirably large and cumbersome hand tool, the device has the disadvantage of measuring the gap at a single point per insertion. Single point measurement is undesirable because at least three gap measurements taken at very precisely located, spaced-apart positions, which are precisely located with respect to the edges of the gap, are required in order to obtain all of the information needed to determine the planar profile of a shim. More specifically, three gap measurements at precise locations are needed because an interface gap, which is three dimensional, can taper in two directions. A single point electrochemical measuring device is undesirable because it must be manually moved to three precise positions in order to obtain the three measurements. Not only is precise manual positioning expensive because it is time consuming, the multiple manual positioning of a single point measuring device is more likely to result in errors than is the single manual positioning of a multiple point measuring device.
Microwave, profilometer and optical measurement devices have also been proposed to measure the thickness of interface gaps that need to be shimmed. As with the mechanical system described above, all of these proposals have the disadvantage of providing measurements at a single location per measurement. Because of the previously described difficulties associated with single location measurement devices, it is difficult to utilize the information generated by such devices to control a machine tool system designed to automatically create a shim based on precise measurement information. In addition, many of the prior art gap measuring devices are relatively bulky, making them unuseful when the gap whose thickness is to be measured is located near adjacent structure.